PI-2177: Add tech docs for CAS2 and Delius (#3927)

projects/cas2-and-delius/README.md

@@ -1,3 +1,47 @@
-# cas2-and-delius
+# CAS2 and Delius
 
-// TODO Describe the service
+Service that responds to Community Accommodation Services Tier 2 (CAS2) domain events. The service is responsible for updating Delius with progress of referrals to short-term accommodation for people released from custody on Home Detention Curfew or bail.
+
+## Business Need
+
+Ensuring the Probation Practitioner has an up-to-date view of activity in the CAS2 service when interacting with the probation case via Delius.
+
+## Context Map
+
+![Context Map](./tech-docs/src/img/cas2-and-delius-context-map.svg)
+
+## Workflows
+
+CAS2 domain events are raised in real time as referral applications are processed. There is a single update domain event to inform of progress or changes to the referral application.
+
+### Create a CAS2 Referral Application
+
+A referral application is created in the CAS2 service which is reflected in a Delius contact. An `EXTERNAL_REFERENCE` URN is added to the contact in the form `urn:hmpps:cas2:application-submitted:{referralId}` using the CAS2 referral ID as a unique reference.
+
+| Business Event                                                | Message Event Type / Filter                  |
+|---------------------------------------------------------------|----------------------------------------------|
+| Submission of a accommodation referral application            | applications.cas2.application.submitted      |
+
+![Context Map](./tech-docs/src/img/cas2-and-delius-workflow-application.svg)
+
+### Update the Status of a CAS2 Referral Application
+
+A referral application is updated in the CAS2 service which is reflected in a Delius contact. Only a single update contact exists in Delius for any single CAS2 referral and the contact is updated with the latest status. An `EXTERNAL_REFERENCE` URN is added to the contact in the form `urn:hmpps:cas2:application-status-updated:{referralId}` using the CAS2 referral ID as a unique reference.
+
+| Business Event                                                | Message Event Type / Filter                  |
+|---------------------------------------------------------------|----------------------------------------------|
+| Update to the status of an accommodation referral application | applications.cas2.application.status-updated |
+
+![Context Map](./tech-docs/src/img/cas2-and-delius-workflow-application-update.svg)
+
+## Interfaces
+
+### Message Interface
+
+The service responds to HMPPS Domain Event messages via the
+[Queue](https://github.com/ministryofjustice/cloud-platform-environments/blob/main/namespaces/live.cloud-platform.service.justice.gov.uk/hmpps-probation-integration-services-prod/resources/cas2-and-delius-queue.tf).
+The events are raised by the [HMPPS Approved Premises API](https://github.com/ministryofjustice/hmpps-approved-premises-api/) which handles backend integration for the [CAS2 UI](https://github.com/ministryofjustice/hmpps-community-accommodation-tier-2-ui)
+
+Example [messages](./src/dev/resources/messages/) are in the development source tree.
+
+Incoming messages are filtered on `eventType` by the [SQS queue policy](https://github.com/ministryofjustice/cloud-platform-environments/blob/2aeb8aeb7b7798cbe12bc81b14d01aaa707041f1/namespaces/live.cloud-platform.service.justice.gov.uk/hmpps-probation-integration-services-prod/resources/cas2-and-delius-queue.tf#L5-L10)

projects/cas2-and-delius/tech-docs/diag/cas2-and-delius-context-map.d2

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+
+style { ...@../../../../script/style.style }
+vars { ...@../../../../script/style.vars }
+** { ...@../../../../script/style.all-style }
+
+context-cas2: CAS2 Context {
+  entities: Entities {
+    grid-columns: 2
+    pop: Person on Probation
+    referral: Accommodation Referral
+  }
+}
+
+context-delius: Delius Context {
+
+  interface: CAS2 and Delius {
+    type: Anti-Corruption\nLayer
+  }
+
+  entities: Entities {
+    grid-columns: 2
+    pop: Person on Probation
+    contact: Contact
+    manager: Offender Manager
+  }
+}
+
+context-cas2--context-delius.interface: Upstream\n\n\nDownstream

projects/cas2-and-delius/tech-docs/diag/cas2-and-delius-workflow-application-update.d2

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+# CAS2 Workflow: Referral Application Status Update
+
+style { ...@../../../../script/style.style }
+vars { ...@../../../../script/style.vars }
+** { ...@../../../../script/style.all-style }
+
+direction: right
+
+# What triggers the workflow?
+cas2: CAS2  {
+     style.font-size: 24
+
+     explanation: |md
+     ## Referral Application Workflow
+     Updates to referral applications raise domain events
+     |
+
+     application_updated: Application Updated
+}
+
+domain_event_1: "applications.cas2\n.application.status-updated" {
+    style.font-size: 12
+    shape: queue
+}
+
+# What happens in Delius?
+delius: Delius {
+    style.font-size: 24
+
+     explanation: |md
+     ## Delius Workflow
+     Referral application status contact is created\
+     or updated Delius to reflect referral application changes
+     |
+
+    d_contact_create: Create Contact
+    d_contact_update: Update Contact
+}
+
+cas2 -> domain_event_1
+
+domain_event_1 -> delius

projects/cas2-and-delius/tech-docs/diag/cas2-and-delius-workflow-application.d2

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+# CAS2 Workflow: Referral Application
+
+style { ...@../../../../script/style.style }
+vars { ...@../../../../script/style.vars }
+** { ...@../../../../script/style.all-style }
+
+direction: right
+
+# What triggers the workflow?
+cas2: CAS2  {
+     style.font-size: 24
+
+     explanation: |md
+     ## Referral Application Workflow
+     Referral application actions raise domain events
+     |
+
+     application_submitted: Application Submitted
+}
+
+domain_event_1: "applications.cas2\n.application.submitted" {
+    style.font-size: 12
+    shape: queue
+}
+
+# What happens in Delius?
+delius: Delius {
+    style.font-size: 24
+
+     explanation: |md
+     ## Delius Workflow
+     Contact is added to Delius \
+     to indicate referral application creation
+     |
+
+    d_contact: Create Contact
+}
+
+cas2 -> domain_event_1
+
+domain_event_1 -> delius